Multi-layer coating systems are well known in the coatings industry. One area in which multi-layer coating systems have become increasingly utilized is the area of decorative and protective coatings for automotive vehicles. A common multi-layer coating system for automotive application is the so-called "base coat/top coat" system in which a thicker layer of clear, unpigmented topcoat is applied over an underlying thinner layer of pigmented base coat. The base coat may contain reflective particulate flake materials such as aluminum flake or mica flake to impart "sparkle" to so-called metallic finishes.
These base coat/top coat coating systems may comprise either "wet-on-wet" one-bake systems in which the clear top coat layer is applied over an unbaked base coat layer with both layers being cured in a single subsequent bake curing step, or "multiple-bake" systems in which a clear coat is applied and cured over previously applied and cured base coats. Modern multi-layer coatings systems are typically of the wet-on-wet one-bake type which eliminate or minimize problems of adherence between the layers which may sometimes result with multiple-bake systems.
Early base coat/top coat coating systems for automotive applications generally comprised solvent-borne, pigmented base coat compositions and solvent-borne clear coat compositions. However, rising costs of solvents and the growing concern over the environmental impact of organic solvents has forced research efforts in recent years to turn toward means for eliminating or minimizing the organic solvent content of coating compositions.
A certain degree of success has been achieved in lowering the total volatile organic content (VOC) of multi-coat systems by reducing the organic content of both the base coat and top coat compositions. For example, recent developments in multilayer coating systems have seen the introduction of water-borne base coat compositions. One example of such water-borne systems is disclosed in U.S. Pat. No. 4,794,147 which employs as principal resin and/or pigment grind resin a water-dispersible polyurethane resin. Coating formulations based upon these water-borne polyurethane resins can generally be applied to substrates over a broader range of ambient temperature and humidity conditions. Another type of water-borne system is exemplified by anionically stabilized acrylic resins of the type disclosed in U.S. Pat. No. 4,403,085 and 4,518,724. Acrylic resins are generally characterized by good durability and weatherability, but sometimes suffer from the shortcoming that they must be applied within a narrow range of temperatures and humidities to avoid problems of flow and sag.
Anionically stabilized acrylic resins are typically formed by the polymerization of a mixture of acrylic monomers which contains a certain fraction of carboxyl group-containing monomers such as acrylic acid or methacrylic acid. The resin is dispersed in water and stabilized in aqueous medium by conversion of at least a portion of the carboxyl groups to carboxylate salt groups by neutralization with a base, typically a tertiary amine.
Present-day commercially viable clear top coats typically are based upon alkylated melamine resins which are thermally cross-linked in the presence of one or more acid catalysts well known in the art. Acrylic resins which are cross-linked by melamine resins are presently the resins of choice for top coats because of their exceptional gloss and durability, qualities which are highly desirable for the clear top coat in a multilayer automotive coating system. A thorough discussion of so-called "monomeric" highly alkylated melamine resins and "polymeric," less highly alkylated melamine resins can be found in Kirsch, Albert J., "50 Years of Amine Coating Resins," the Winchell Co., Philadelphia, Pa., 1986, the contents of which are incorporated herein by reference.
The highly alkylated melamine resins generally require strong acid catalysis for curing, while the less highly alkylated melamines can be cured by the use of weaker, general acid catalysts. However, in either case, organic solvents are usually required to prepare melamine-containing clear coat formulations. The highly alkylated monomeric melamine resins typically form less viscous solutions in organic solvents, permitting higher solids loading, with attendant reduction in VOC. The less-alkylated, polymeric melamine resins, on the other hand, form more viscous solutions and cannot generally be employed to form high solids clear coat formulations.
With the introduction of acrylic anionically stabilized water-borne base coat compositions, and the desire to use high solids clear coats to reduce VOC, compatibility problems have arisen. The preferred highly alkylated monomeric melamine top coat resins, requiring strong acid catalysis for curing are generally not entirely compatible with anionically stabilized base coat resins. In wet-on-wet one-bake systems where the base coat resins are of the amine-neutralized carboxyl function-containing type, amine which is freed from the base coat layer during the baking step often neutralizes a portion of the strong acid catalyst employed to cure the highly alkylated melamine resins in the top coat. This can sometimes result in an incompletely cured top coat layer with attendant reduction in gloss, distinctness of image, durability and weatherability. On the other hand, if a bake curing step is imposed between application of the base coat and clear coat layers to eliminate this problem, intercoat adherence may suffer.
There is thus a need in the art for acrylic-based resins which can be employed in base coat paint formulations, which can be readily dispersed and stablizied in water, and which are compatible with high solids, highly alkylated melamine type top coat formulations.